This invention relates to a converter circuit for a digital line repeater, which may be described more particularly as a converter for producing a precision N-level signal.
Digital transmission lines have been developed for a variety of transmission media. In particular, three systems, T1, T1C and T2 have been designed to operate on wire pairs.
T1 is a twenty-four channel system that operates at 1.544 Mb/s on exchange grade cable. Maximum repeater spacing is in excess of 6000 feet on 22-gauge copper pairs. The T1 system is limited to 50 miles in length.
T1C is a forty-eight channel system that operates at 3.152 Mb/s on pulp, PIC and MAT exchange grade cable. Maximum repeater spacing is greater than 6000 feet on 22-gauge copper pairs, and the sytem is limited to 50 miles.
T2 is a ninety-six channel system which operates at 6.312 Mb/s on a low-capacitance PIC cable. Maximum repeater spacing is 14,800 feet. The T2 system is used on intercity routes as long as 500 miles.
Growth in the use of digital transmission facilities has stimulated needs for new plans and new system designs. For instance many short system routes, which have exchange grade cable arranged for T1C systems, are reaching the capacity of their cables.
Additional digital transmission facilities can be furnished by laying another cable or by increasing the capacity of the existing cable. The installation of such a new cable often is extremely expensive. An attractive alternative is to increase the capacity of the electronics operating in the existing cable without adding any new cable. The capacity of the electronics can be increased from forty-eight channels to ninety-six channels by transmitting an unrestricted 4-level pulse-amplitude modulated signal rather than the standard bipolar signal used in the T1C system.
A new converter is required to produce the 4-level signal to be transmitted along the wire pairs in the cable. Heretofore a converter, for producing the 4-level signal includes transistors of opposite conductivity types and bias sources of both positive and negative polarities. Mismatches between the devices and between the bias sources cause operational problems which are either difficult or costly to control in systems having the converters deployed in repeater circuits in the uncontrolled environment along the cable route.